Oil refineries and storage terminals utilize floating roof tanks for the storage of hydrocarbon stocks which have a high vapour pressure. Typical products are gasoline, naphthas and crude oil.
The filling and emptying of the tanks is between a normal minimum to a normal maximum gauge (or depth) which typically is approximately 2 meters to approximately 14 meters respectively. The minimum gauge elevation is determined by the need to keep the underside of the roof clear of any projections into the tank (e.g. tank heaters, mixers, suction/rundown lines) and the requirement to provide sufficient head for pumping equipment connected to the tank. As all working movements in the tank are above the minimum gauge, the volume in the tank at minimum gauge, (or heel) is a static inventory which represents the high cost of working capital inventory. This liquid must be purchased but cannot be sold as it cannot be extracted from the tank while the tank remains in use.